MEMS integrated pressure sensor devices and methods of forming same

ABSTRACT

A method embodiment includes providing a micro-electromechanical (MEMS) wafer including a polysilicon layer having a first and a second portion. A carrier wafer is bonded to a first surface of the MEMS wafer. Bonding the carrier wafer creates a first cavity. A first surface of the first portion of the polysilicon layer is exposed to a pressure level of the first cavity. A cap wafer is bonded to a second surface of the MEMS wafer opposite the first surface of the MEMS wafer. The bonding the cap wafer creates a second cavity comprising the second portion of the polysilicon layer and a third cavity. A second surface of the first portion of the polysilicon layer is exposed to a pressure level of the third cavity. The first cavity or the third cavity is exposed to an ambient environment.

This application claims the benefit of U.S. Provisional Application No. 61/784,115, filed on Mar. 14, 2013, entitled “MEMS Pressure and Motion Sensor Devices and Methods of Forming Same,” which application is hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following co-pending and commonly assigned patent applications filed on the same date: “MEMS Integrated Pressure Sensor and Microphone Devices and Methods of Forming Same” Ser. No. 13/944,382; “MEMS Integrated Pressure Sensor Devices having Isotropic Cavities and Methods of Forming Same” Ser. No. 13/906,105; “MEMS Integrated Pressure Sensor and Microphone Devices having Through-Vias and Methods of Forming Same” Ser. No. 13/955,957; and “MEMS Device and Methods of Forming Same” Ser. No. 13/893,058.

BACKGROUND

Micro-electromechanical systems (“MEMS”) are becoming increasingly popular, particularly as such devices are miniaturized and are integrated into integrated circuit manufacturing processes. MEMS devices introduce their own unique requirements into the integration process, however. Electrically interconnecting MEMS devices is an area of unique challenges. In particular, integrating MEMS pressure sensor devices with other MEMS devices (e.g., motion sensor devices) into the same integrated circuit manufacturing process has posed challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1A-1Z are cross-sectional views of intermediate stages of manufacture of a MEMS device in accordance with various embodiments;

FIGS. 2A-2I are cross-sectional views of intermediate stages of manufacture of a MEMS device in accordance with various alternative embodiments;

FIG. 3 is a top-down view of a portion of a MEMS wafer in accordance with various embodiments; and

FIGS. 4A-4C are varying cross-sectional and top-down views of a MEMS wafer in accordance with various alternative embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosed subject matter, and do not limit the scope of the different embodiments.

FIGS. 1A-1Z illustrate cross-sectional views of intermediate stages of manufacture of a portion of a MEMS device 100 having a pressure sensor 404 and another device 406 (see FIG. 1Z). Device 406 may be a MEMS motion sensor, a gyroscope, an accelerometer, or the like. Pressure sensor 404 and device 406 are manufactured using the same integrated circuit (IC) process to create sealed cavities (i.e., cavities 408 and 410) and an ambient environment opening (i.e., opening 402) in MEMS device 100. Therefore, various embodiments illustrated by FIGS. 1A-1Z allow for the smooth integration of manufacturing a MEMS pressure sensor device using known IC manufacturing techniques.

As shown in FIG. 1A, MEMs device 100 includes a substrate 102 and a dielectric layer 104, referred to as oxide release layer 104. Substrate 102 may be formed of silicon, or other materials such as silicon germanium, silicon carbide, or the like. Substrate 102 may be formed of low resistive silicon. Alternatively, substrate 102 may be a silicon-on-insulator (SOI) substrate. SOI substrate may comprise a layer of semiconductor material (e.g., silicon, germanium, and the like) formed over an insulator layer (e.g., buried oxide), which is formed in a silicon substrate. In addition, other substrates that may be used include multi-layered substrates, gradient substrates, hybrid orientation substrates, and the like.

Oxide release layer 104 may be formed of a low-k dielectric material, such as silicon dioxide (SiO₂). Oxide release layer 104 may be deposited over substrate 102 using, for example, spinning, chemical vapor disposition (CVD), plasma enhanced chemical vapor deposition (PECVD), low pressure CVD, thermal oxidation, or other suitable deposition techniques as are known in the art. Furthermore, oxide release layer 104 may be formed of a different suitable material such as low-k dielectrics such as carbon doped oxides, extremely low-k dielectrics such as porous carbon doped silicon dioxide, a polymer such as polyimide, combinations thereof, or the like. Oxide release layer 104 may be released (i.e., removed) in subsequent process steps in order to form MEMS structures; therefore, oxide release layer 104 may also be referred to as sacrificial (SAC) oxide layer 104 or sacrificial layer 104.

FIG. 1B illustrates the patterning of oxide release layer 104 to include openings 106. This may be done, for example, using a combination of photolithography and etching techniques. In FIG. 1C, upper portions of openings 106 are seamed together, sealing openings 106, which may now be referred to as voids 106. Openings 106 may be seamed together using, for example, an oxide deposition process applied to the upper surface of oxide release layer 104. For example, the deposition of additional oxide material over oxide release layer 104 may be employed to seal off the upper portions of openings 106. The oxide deposition may be formed using a deposition process such as CVD or the like. More particularly, by controlling the deposition process, the material of oxide release layer 104 may be deposited in a non-conformable manner. That is, the material of oxide release layer 104 may build up on the upper portions of openings 106 faster than along the sidewalls and bottom of openings 106. This process leads to the formation of an overhang at the edge of the upper portion of opening 106, as the deposition process continues, the overhangs will merge, sealing off opening 106 with a plurality of seams and forming voids 106.

Voids 106 may be included in oxide release layer 104 to decrease release time in subsequent process steps. That is, the inclusion of voids 106 creates weak spots in oxide release layer 104 that reduces releasing time of MEMS structures. Alternatively, the steps shown in FIGS. 1B and 1C may be omitted if release time speed is not a concern, or an alternative design for a MEMS device includes different methods of decreasing release time.

In FIGS. 1D and 1E, oxide release layer 104 is patterned, creating bump openings 108 and via openings 110. The patterning of oxide release layer 104 may be done using, for example, a combination of photolithography and etching techniques. Two separate photolithography steps may be performed in order to create bump openings 108 and via openings 110. For example, a shallow etching may be performed to create bump openings 108, while a deeper etching may be performed to create via openings 110. As shown in FIGS. 1D and 1E, bump openings 108 do not extend to substrate 102 while via openings 110 do extend to and expose portions of substrate 102. Furthermore, oxide release layer 104 may be thinned (not shown) until a desired thickness is achieved. The thinning process may be implemented by using suitable techniques such as grinding, polishing, and/or chemical etching. For example, a chemical mechanical polishing (CMP) process may be used to thin oxide release layer 104. In accordance with various embodiments, the thickness of oxide release layer 104 is in a range from about 0.5 μm to about 5 μm.

In FIG. 1F, a dielectric layer 112 is deposited over oxide release layer 104 using, for example, CVD. Dielectric layer 112 may be made of polysilicon, and may be referred to as polysilicon layer 112 hereinafter. Polysilicon layer 112 fills via openings 110 and bump openings 108, forming polysilicon vias 112A and polysilicon bumps 112B respectively. Polysilicon vias 112A may be formed for electrical routing and may further act as a mechanical structure. For example polysilicon vias 112A may be used as a vapor hydrogen-fluoride (vapor HF) etch stop layer in subsequent process steps. Furthermore, in some embodiments, polysilicon bumps 112B may be used as mechanical bumps to limit the motion of moving elements in MEMS device 100, or anti-stiction bumps. In alternative embodiments, layer 112 may be formed of a different material in lieu of polysilicon such as SiGe, single crystal silicon (e.g., by using a silicon-on-insulator wafer as a starting material), and the like. It should be noted that while a single polysilicon layer is illustrated, those skilled in the art will recognize that multiple polysilicon layers could be employed.

An oxide mask layer 114 may be formed and patterned over a portion of polysilicon layer 112. Oxide mask layer 114 is formed out of a similar material and using similar methods as oxide release layer 104, and oxide mask layer 114 may be patterned using for example, a combination of photolithography and etching. Oxide mask layer 114 acts as protection for critical portions of polysilicon layer 112 in subsequent process steps. For example, in FIG. 1F, oxide mask layer protects portions of polysilicon layer 112 to ensure proper thickness control and surface texture. Oxide mask layer 114 may be formed over any portion of polysilicon layer 112 where such control is desired. If surface texture and thickness is not crucial, oxide mask layer 114 may be omitted.

In FIG. 1G, polysilicon layer 112 is patterned using for example a combination of photolithography and etching. The patterning of polysilicon layer 112 may be done in order to create portions of various devices in MEMS device 400. For example, the patterning of polysilicon layer 112 in FIG. 1G creates separate portions of polysilicon layer 112 for inclusion in as a bottom electrode of a motion sensor (or other applicable device) and a membrane of a pressure sensor respectively.

In FIG. 1H, etch stop layer 116 is formed and patterned over oxide release layer 104 and polysilicon layer 112. Etch stop layer 116 may be deposited using, for example, low pressure chemical vapor deposition (LPCVD), and etch stop layer 116 may be patterned using, for example, a combination of photolithography and etching. Etch stop layer 116 is patterned to include release holes 117 and to expose portions of oxide mask layer 114. Release holes 117 provide a path to remove portions of oxide release layer 104 in subsequent process steps. Etch stop layer 116 may be used as a vapor HF etch stop layer and may be formed of a low-stress nitride (LSN). However, other materials such as aluminum nitride, aluminum oxide, silicon carbide, or other dielectrics chemically resistant to vapor HF may be used as well.

FIG. 1I illustrates the formation and patterning of another oxide release layer 118. Oxide release layer 118 formed of substantially the same material and using substantially the same methods as oxide release layer 104. The thickness of oxide release layers 118 and 104 may be designed to control parasitic feedback through capacitance and/or the gap between the subsequent movable element of MEMS device 100 and the polysilicon layer 122 (see FIG. 1J). Oxide release layer 118 may be blanket deposited and then undergo a thinning process (e.g., CMP or etch-back) to reach a desired flatness and/or thickness. Oxide release layer 118 is patterned to create via openings 120 using for example, a combination of photolithography and etching.

FIG. 1J illustrates the formation of a thin polysilicon layer 122 over oxide release layer 118. Thin polysilicon layer 122 may formed on oxide release layer 118 using suitable techniques such as CVD, and the like. Thin polysilicon layer 122 is deposited into via openings 120, creating via portions 122A connected to polysilicon layer 112. Thin polysilicon layer 122 acts as electrical routing (e.g., using via portions 122A). Thin polysilicon layer 122 also acts as a shield for various components of MEMS device 100 (e.g., substrate 102 and polysilicon layer 112) in subsequent process steps. Thin polysilicon layer 122 also acts as a bonding interface layer; therefore, other suitable bonding materials such as silicon, amorphous silicon, silicon doped with impurities, combinations thereof, and the like may be used in lieu of polysilicon.

In FIG. 1K, portions of thin polysilicon layer 122 and oxide release layer 118 are etched, creating openings 124. This may be done using, for example, a combination of wet and dry etching techniques. Notably, one of the openings 124 (124A) exposes region 128 of polysilicon layer 112. Region 128 of polysilicon layer 112 may act as a membrane of a pressure sensor device in completed MEMS device 100 (e.g., see element 404 in FIG. 1Z). In a completed MEMS device 100, opening 124A exposes this portion of polysilicon layer 112 to a certain level of pressure (e.g., ambient pressure or sealed pressure depending on the design of MEMS device 100). The etching of thin polysilicon layer 122 and oxide release layer 118 completes a MEMS wafer 126 of MEMS device 100. MEMS wafer 126 has a top and bottom surface, 126A and 126B respectively.

In FIG. 1L, MEMS wafer 126 is bonded to carrier wafer 200. FIG. 1M illustrates a more detailed view of a carrier wafer 200. Carrier wafer 200 may be a substrate that is substantially similar to substrate 102 in MEMS wafer 126. Carrier wafer 200 is patterned to include openings 201 using for example etching techniques adopting an etching loading effect. Carrier wafer 200 also has a top and bottom surface, 200A and 200B respectively. Top surface 126A of MEMS wafer 126 is bonded to top surface 200A of carrier wafer 200.

Openings 124 of MEMS wafer 126 may be aligned to openings 201 of carrier wafer 200. MEMS wafer 126 may be bonded to carrier wafer 200 using any suitable technique such as fusion bonding, anodic bonding, eutectic bonding, and the like. For example, in various embodiments, MEMS wafer 126 may be fusion bonded to carrier wafer 200 using thin polysilicon layer 122 as a bonding interface.

Furthermore, MEMS wafer 126 may be thinned to a desired thickness T3. The thinning process may include grinding and chemical mechanical polishing (CMP) processes, etch back processes, or other acceptable processes performed on surface 126B of MEMS wafer 126 (i.e., substrate 102). As a result of this thinning process, MEMS wafer 126 may have a thickness between about 5 μm to about 60 μm.

In FIG. 1N, conductive bonds 202 are formed and patterned over substrate 102 (i.e., bottom surface 126B of MEMS wafer 126). Conductive bonds 202 may be formed of aluminum copper (AlCu) and are used for eutectic bonding in subsequent process steps. Alternatively, a different conductive material suitable for eutectic bonding such as Ge, Au, combinations thereof, or the like may be used instead.

In FIG. 1O, portions of substrate 102 are patterned forming openings 204 using for example, a combinations of photolithography and etching. The portions of the remaining substrate 102 may form various MEMS structures (e.g., MEMS structures 206 and 208). MEMS structure 206 may act as a bottom electrode of a pressure sensor device in finished MEMS device 100. MEMS structure 208 may be patterned to act as a proof mass of a motion sensor device in finished MEMS device 100. Alternatively, MEMS structure 208 may also be patterned to be portions of other MEMS devices such as a spring (e.g., for a gyroscope), a series of fingers in a comb (e.g., for an accelerometer), or the like.

FIG. 1P illustrates the release of the MEMS structures 206 and 208 by a vapor HF etching of portions of oxide release layers 104 and 118. This type of etch process has a high selectivity between oxide release layers 104 and 118, etch stop layer 116, polysilicon layer 112, thin polysilicon layer 122, and carrier wafer 200 so that that polysilicon layers 112 and 122, carrier wafer 200, and etch stop layer 116 are not significantly attacked during the removal of portions of oxide release layers 104 and 118. Furthermore, polysilicon layer 112 (e.g., vias 112A) and etch stop layer 116 protects portions of oxide release layers 104 and 118 during the etch process, and these protected regions may be referred to as anchor regions. This etch process allows for free movement of the movable elements of MEMS structure 208 in at least one axis. Furthermore, MEMS structure 206 may be designed to be stiff and having a relatively limited range of motion even after the vapor HF process. It should be noted that the portions of the oxide release layers to be removed depend on layout design.

FIGS. 1Q-1V illustrate various intermediary steps of manufacture of a cap wafer 300 for inclusion in the completed MEMS device 100. Cap wafer 300 may or may not be a semiconductor wafer (e.g., a CMOS wafer), which may or may not have electrical circuits (not shown). In particular cap wafer 300 may include various active devices such as transistors, capacitors, resistors, diodes, photodiodes, fuses and the like. The electrical circuits may be interconnected to perform one or more functions suitable for a particular application, which may or may not be related to MEMS device 100. FIG. 1Q illustrates cap wafer 300 as having substrate 302, oxide layer 304, and patterned metal lines 306. Substrate 302 and oxide layer 304 may be substantially similar to substrate 102 and oxide layer 104 in MEMS wafer 126. Metal lines 306 may be formed of aluminum copper (AlCu) and may be used for electrical routing. Alternatively, metal lines 306 may be formed of another suitable metallic material.

In FIG. 1R, a conforming oxide layer 308 is formed over metal lines 306. Conforming oxide layer 308 may be deposited using any suitable technique, such as CVD, and the like and may be a low-k dielectric material. The formation of conforming oxide layer 308 may include a grinding process (e.g., CMP) to achieve a desired topography and thickness. A film layer 310 is deposited over conforming oxide layer 308 using a suitable technique such as CVD. In some embodiments, film layer 310 is formed of silicon nitride and is used as a passivation layer. Alternatively, film layer 310 may be formed of a dielectric material such as an oxide, a metal, combinations thereof, or the like. In subsequent process steps, portions of film layer 310 may be patterned to crate mechanical bumps in cap wafer 300.

FIG. 1S illustrates insertion of contact plugs 311 into cap wafer 300. Contact plugs 311 may be formed of tungsten, although other metallic materials such as aluminum or copper may also be used. Contact plugs 311 may be formed for example, by patterning film layer 310 and conforming oxide layer 308, exposing metal lines 306. A metallic material, e.g., tungsten, may be deposited in the patterned openings and a CMP technique may be used so that the top surface of contact plugs 311 may be level with the top surface of film layer 310. Contact plugs 311 are electrically connected to metal lines 306.

In FIG. 1T, bonding material layers 312 (alternatively referred to as bonds 312) are formed over a top surface of film layer 310. Bonding material layers 312 may be blanket deposited and patterned using for example physical vapor deposition (PVD) and photolithography/etching. Bonding material layers 312 may be made of a layer of aluminum copper under a layer of germanium although other metallic materials such as gold may also be used. Bonding material layers 312 may act as a eutectic bonding material for a subsequent bonding process. Bonding material layers 312 may or may not be electrically connected to metal lines 306 via contact plugs 311.

In FIG. 1U, a shallow etching is performed on portions of film layer 310. Portions of film layer 310 may be shallow etched to facilitate the exposure of portions of metal line 306 in a subsequent process step. Furthermore, the etching of film layer 310 may form bumps 314. Bumps 314 may serve a variety of purposes. For example, in an embodiment bumps 314 are mechanical bumps included to limit the motion of moving elements in MEMS device 100. Bumps 314 may also be used as anti-stiction bumps.

In FIG. 1V, a cavity 316 is formed in cap wafer 300. Cavity 316 may function as a sealed cavity in a pressure sensor device of MEMS device 100. The formation of cavity 316 may include known etching techniques.

FIG. 1W illustrates the stacked MEMS device 100, wherein cap wafer 300 is stacked over MEMS wafer 126 and carrier wafer 200. Cap wafer 300 may be bonded to MEMS wafer 126 by eutectic bonding between the bonds 202 and bonds 312. As shown in FIG. 1W, through the eutectic bonding process, moveable elements (e.g., MEMS structure 206 and 208) may be located between a polysilicon layer 112 and cap wafer 300. Furthermore, Cap wafer 300 and MEMS wafer 126 are aligned so that cavity 316 and bumps 314 are aligned with MEMS structure 206 and MEMS structure 208 respectively. MEMS structures 206 and 208 are disposed in sealed cavities defined by the eutectic bonding. That is, in a top-down view of portions of MEMS device 100 (see FIG. 3), at least a portion of the eutectic bonds formed between bonds 202 and 312 form closed loops, sealing MEMS structures 206 and 208 in enclosed cavities.

In FIG. 1X, a grinding process is performed to remove portions of MEMS wafer 126 and carrier wafer 200. The grinding may also be referred to as an open pad grinding (OPG) exposing portions of cap wafer 300 and may be done using known grinding techniques. The OPG may be facilitated by the inclusion of openings 201 in carrier wafer 200 (see FIG. 1W). That is, portions of MEMS wafer 126 and carrier wafer 200 may be easily removed by removing a small portion of carrier wafer 200 (defined by the placement of openings 201). In FIG. 1Y, portions of film layer 310 and conforming oxide layer 308 may also be removed (e.g., using dry etch) to expose portions of metal lines 306. These exposed portions of metal line 306 (i.e., portions 306A and 306B) may be used as input/output pads to electrically couple circuits in cap wafer 300 to external circuits (not shown).

In FIG. 1Z, portions of carrier wafer 200 may be removed to expose cavity 402 to ambient pressure. That is, cavity 402 is exposed to an open air environment. The removal of portions of carrier wafer 200 may include known etching techniques such as CMP, etch-back, or the like.

FIG. 1Z illustrates a completed MEMS device 100 in accordance with various embodiments. MEMS device 100 includes a pressure sensor 404 and a device 406. Pressure sensor 404 includes a membrane (i.e., region 128 of polysilicon layer 112) and an electrode (i.e., MEMS structure 206). The membrane is exposed to ambient pressure on one surface (e.g., through cavity 402) and sealed pressure on the other surface (e.g., through sealed cavity 408). The pressure of sealed cavity 408 may be defined by the conditions of the eutectic bonding process between MEMS wafer 126 and cap wafer 300. For example, the eutectic bonding process may be performed in chamber having a certain pressure level to define an appropriate pressure level of sealed cavities 408 and 410 (explained in greater detail below). Therefore, pressure sensor 404 may detect ambient pressure by comparing the difference between cavity 402 and sealed cavity 408.

Device 406 may be a motion sensor that allows for the detection of motion through the disposition of a proof mass (i.e., MEMS structure 206) over an electrode (i.e., portions of polysilicon layer 112) in a sealed cavity 410 having pressure defined by eutectic bonding. Alternatively, device 406 may be an accelerometer, a gyroscope, or the like. The pressure of sealed cavity 410 may be selected in accordance with the desired functionality of device 406. For example, sealed cavity 410 may have a pressure between about 100 mbar to about 700 mbar for an accelerometer, between about 10⁻⁴ mbar to about 10 mbar for a gyroscope, or the like. Thus, using the various formation steps illustrated in FIGS. 1A-1Z, a pressure sensor and another MEMS device may be formed using the same MEMS manufacturing process.

FIGS. 2A-2I illustrate cross-sectional views of various stages of manufacture of a MEMS device in accordance with alternative embodiments.

In FIG. 2A, a cross-sectional view of a carrier wafer 500 is shown. Carrier wafer 500 includes a substrate 502 and a dielectric layer 504. Substrate 502 may be a silicon substrate, while dielectric layer 504 may be a thermal oxide layer formed by performing a thermal oxidation on carrier wafer 500. Dielectric layer 504 may be patterned using for example a combination of photolithography and etching.

In FIG. 2B, portions of substrate 502 and dielectric layer 504 are patterned using for example a combination of photolithography and etching.

In FIG. 2C, deep cavities 506 and 508 are formed using dielectric layer 504 as a mask. Deep cavities 506 and 508 may be formed using, for example, a deep-reactive-ion etching (DRIE) process on substrate 502. Alternatively, if deep cavities are not required in the design of a MEMS device, dielectric layer 504 and the deep etching step may be omitted, and substrate 502 may be patterned and etched using other techniques (e.g., photolithographic techniques).

FIG. 2D illustrates carrier wafer 500 bonded to a MEMS wafer 600. Carrier wafer 500 may be bonded to MEMS wafer 600 using known bonding techniques such as fusion bonding, and the like. MEMS wafer 600 may be substantially similar to MEMS wafer 126 and may be formed using substantially similar process steps. Specifically, MEMS wafer 126 includes a substrate 602, oxide release layers 604, etch stop layer 606, polysilicon layer 608, and thin polysilicon layer 610. Therefore, extensive description on the formation of MEMS wafer 600 is omitted for brevity. However, MEMS wafer 600 includes a trench 617 in area 619B of oxide release layer 604, which may be more readily visible from a top down view of MEMS wafer 600 (see FIG. 4A illustrating a cross-sectional and corresponding top-down view of MEMS wafer 600). As explained in greater detail in subsequent paragraphs, trench 617 helps form a leak path to ambient environment in the completed MEMS device 800. Notably, polysilicon layer 608 (shown in ghost in FIG. 4A's top-down view of MEMS wafer 600) may be patterned so as to not obstruct trench 617.

Thin polysilicon layer 610 may be used as a bonding layer to bond MEMS wafer 600 to carrier wafer 500. Furthermore, polysilicon layer 608 includes portion 608A which is aligned and exposed to deep cavity 508. In the completed MEMS device, portion 608A of polysilicon layer 608 may function as a membrane of a pressure sensor device.

In FIG. 2E, conductive bonds 612 are formed and patterned over substrate 602 (i.e., surface 600A of MEMS wafer 600). Conductive bonds 612 may be formed of aluminum copper (AlCu) and are used for eutectic bonding in subsequent process steps. Alternatively, a different conductive material suitable for eutectic bonding such as Ge, Au, combinations thereof, or the like may be used instead.

In FIG. 2F, portions of substrate 602 are patterned using for example, a combinations of photolithography and etching. The portions of the remaining substrate 602 may form various MEMS structures (e.g., structures 614 and 616 in FIG. 2G).

FIG. 2G illustrates the release of the MEMS structures 614 and 616 by a vapor HF etching of portions of oxide release layers 604. This type of etch process has a high selectivity between oxide release layers 604, etch stop layer 606, polysilicon layer 608, thin polysilicon layer 610, and carrier wafer 500 so that that polysilicon layers 608 and 610, carrier wafer 500, and etch stop layer 606 are not significantly attacked during the removal of portions of oxide release layers 604. Furthermore, polysilicon layer 608 protects portions of oxide release layers 604 during the etch process, and these protected regions may be referred to as anchor regions. This etch process allows for free movement of the movable elements of MEMS structures 614 in at least one axis. Furthermore, MEMS structure 616 may be designed to be relatively immobile even after the vapor HF process. It should be noted that the oxide release layers to be removed depend on layout design.

Furthermore, FIG. 2G illustrates the formation of a leak path 618 to ambient pressure due to the vapor HF etch process. Leak path 618 may be formed, for example, by including a trench connecting area 619A and 619C through area 619B in the formation/design of MEMS wafer 600 (see FIG. 4A). That is, a trench may have been included in area 619B between substrate 602 and polysilicon layer 112 that is unobstructed by polysilicon layer 608. After the vapor HF etch process, leak path 618 may be formed allowing for free flow of air between area 619A and 619C (see FIG. 4B illustrating a cross-sectional and corresponding top-down view of MEMS wafer 600 after the vapor HF etch process). Leak path 618 may function similarly to opening 402 in MEMS device 100.

FIG. 2H illustrates the eutectic bonding of MEMS wafer 600 (bonded to carrier wafer 500) to a cap wafer 700. Cap wafer 700 includes film layer 704 (having bumps 704A), cavity 702, and metal bonds 706. Cap wafer 700 is substantially similar to cap wafer 300 in MEMS device 100, and cap wafer 700 may be formed using substantially the same process steps as cap wafer 300. Therefore, detailed description of the process steps for forming cap wafer 700 is excluded for brevity. Bonds 706 are eutectically bonded to bonds 612 of MEMS wafer 600. Cavity 702 and bumps 704A are aligned to portion 608A (i.e., a pressure sensor membrane) and MEMS structure 614 (i.e., a moving element of another device) respectively. Referring to FIG. 4C, certain bonds 706 and 612 form closed loops, creating sealed cavities 702 and 710 defined by eutectic bonding. However, certain other bonds 706 and 612 do not form closed loops and may be included for as electrical connections.

FIG. 2I illustrates completed MEMS device 800 having a pressure sensor 802 and a device 804 (e.g., a motion sensor, accelerometer, gyroscope, or the like). In FIG. 2I, a grinding has been performed on MEMS device 800 in expose input/output pads 806. The grinding may be facilitated by the inclusion of openings 506 (see FIG. 2H) in carrier wafer 500.

Pressure sensor 802 includes a polysilicon membrane (portion 608A of polysilicon layer 608), which is exposed to ambient pressure on one surface and sealed pressure on the other. Cavity 508 acts as a sealed pressure chamber defined, for example, by the fusion bonding process of MEMS wafer 600 to carrier wafer 500. Cavity 702 acts as an ambient pressure chamber, wherein a leak path to ambient pressure is shown, for example, by arrow 618. Because certain eutectic bonds (e.g., bond 810) does not seal cavity 702 (i.e., see FIG. 4C), cavity 702 may be exposed to ambient pressure via leak path 618. Device 804 includes MEMS structure 614 in a sealed cavity 710 disposed between cap wafer 700 and polysilicon layer 608. Polysilicon layer 608 may act as a bottom electrode for device 804. Furthermore, the pressure level of cavity 710 is defined by the eutectic bonding process and may be selected in accordance with the desired functionality of device 804.

Although the present embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method for forming a micro-electromechanical (MEMS) device comprising: providing a MEMS wafer, wherein the MEMS wafer comprises: a sacrificial layer over a MEMS substrate; and a polysilicon layer over the sacrificial layer, wherein the polysilicon layer has a first portion and a second portion; bonding a carrier wafer having a first cavity to the MEMS wafer, wherein bonding the carrier wafer includes aligning the first cavity with the first portion of the polysilicon layer; patterning the MEMS substrate; removing portions of the sacrificial layer to form first and second MEMS structures, wherein the first MEMS structure corresponds to the first portion of the polysilicon layer; bonding a cap wafer to the MEMS wafer, wherein bonding the cap wafer forms a first sealed cavity comprising the first portion of the polysilicon layer and the first MEMS structure, and wherein the second MEMS structure is disposed between the second portion of the polysilicon layer and the cap wafer in a second sealed cavity; and removing portions of the carrier wafer to expose the first cavity to ambient pressure.
 2. The method of claim 1, wherein bonding the cap wafer to the MEMS wafer comprises a eutectic bonding process, wherein a pressure level of the first and second sealed cavities are defined by the eutectic bonding process.
 3. The method of claim 1, wherein bonding the carrier wafer to the MEMS wafer comprises a fusion bonding process.
 4. A method for forming a micro-electromechanical (MEMS) device comprising: providing a MEMS wafer comprising: a MEMS substrate; a first oxide release layer over the MEMS substrate; and a polysilicon layer over the first oxide release layer, wherein the polysilicon layer comprises first and second portions; bonding a carrier wafer to a first surface of the MEMS wafer, wherein bonding the carrier wafer creates a first cavity, and wherein a first surface of the first portion of the polysilicon layer is exposed to a pressure level of the first cavity; providing a MEMS structure aligned with the second portion of the polysilicon layer, wherein providing the MEMS structure comprises patterning the MEMS substrate and removing portions of the first oxide release layer; bonding a cap wafer to a second surface of the MEMS wafer, the second surface of the MEMS wafer being opposite the first surface of the MEMS wafer, wherein bonding the cap wafer creates: a second cavity comprising the second portion of the polysilicon layer and the MEMS structure; and a third cavity, wherein a second surface of the first portion of the polysilicon layer is exposed to a pressure level of the third cavity; and exposing the first or the third cavity to an ambient environment.
 5. The method of claim 4, wherein a pressure level of the second cavity is defined by a bonding process between the cap wafer and the MEMS wafer.
 6. The method of claim 4, wherein exposing the first or the third cavity to an ambient environment comprises forming an opening in the carrier wafer to expose the first cavity to an ambient environment.
 7. The method of claim 4, wherein a pressure level of the first cavity is defined by a bonding process between the carrier wafer and the MEMS wafer, and wherein exposing the first or the third cavity to an ambient environment comprises including a leak path in the MEMS wafer exposing the third cavity to an ambient environment.
 8. The method of claim 4, wherein bonding the cap wafer to the second surface of the MEMS wafer comprises a eutectic bonding process between a first plurality of bonds disposed on the second surface of the MEMS wafer and a second plurality of bonds disposed on a surface of the cap wafer.
 9. The method of claim 8, further comprising providing the cap wafer, wherein providing the cap wafer comprises: providing a semiconductor wafer having a metal line; forming a conforming oxide layer over the metal line; forming a film layer over the conforming oxide layer; and forming the second plurality of bonds over the film layer.
 10. The method of claim 9, further comprising forming contact plugs electrically connecting at least a portion the second plurality of bonds to the metal line.
 11. The method of claim 9, further comprising shallow etching a portion of the film layer to form one or more first bumps, and wherein bonding the cap wafer to the MEMS wafer comprises aligning the one or more first bumps to the MEMS structure.
 12. The method of claim 9, further comprising removing portions of the film layer and the conforming oxide layer expose a portion of the metal line, wherein an exposed portion of the metal line acts as an input/output pad.
 13. The method of claim 4, wherein bonding the carrier wafer to the first surface of the MEMS wafer comprises a fusion bonding process.
 14. The method of claim 4, wherein the cap wafer is a semiconductor wafer comprising active circuits.
 15. The method of claim 4, further comprising performing an opening pad grinding process on the carrier wafer to remove portions of the carrier wafer and bonded portions of the MEMS wafer.
 16. The method of claim 4, wherein bonding the carrier wafer to MEMS wafer comprises using a bonding layer of the MEMS wafer as an interface, and wherein providing the MEMS wafer further comprises: forming an etch stop layer over the polysilicon layer; forming a second oxide release layer over the etch stop layer, forming a bonding layer over the second oxide release layer; and exposing the first portion of the polysilicon layer by removing corresponding portions of the etch stop layer, the second oxide release layer, and the bonding layer.
 17. The method of claim 4, further comprising including voids in the first oxide release layer.
 18. The method of claim 4, wherein removing portions of the first oxide release layer comprises a vapor hydrogen-fluoride (vapor HF) etching process.
 19. The method of claim 4, wherein providing the MEMS wafer further comprises forming the first portion of the polysilicon layer in accordance with a membrane of a pressure sensor.
 20. A method for forming a micro-electromechanical (MEMS) device comprising: providing a MEMS wafer comprising a polysilicon layer; forming a sealed cavity over a first surface of a portion of the polysilicon layer, wherein a first surface of the portion of the polysilicon layer is exposed to a pressure level of the sealed cavity; and exposing a second surface of the portion of the polysilicon layer to an ambient pressure level, wherein forming the sealed cavity and exposing the second surface of the portion of the polysilicon layer comprises bonding a cap wafer and a carrier wafer to opposing surfaces of the MEMS wafer. 